Fatigue Loading for SSI in Shear

[B767]

Good morning,
Trying to come up with a fatigue loading on a bulkhead web(not pressurized). As we know, the web is primarily designed for shear, so the fatigue cracks would be more like Mode II (in-plane shear) or Mode III (out-of-plane shear, transverse shear), but not the favorite Mode I where load is normal to the crack face. So my question is, how to come up with a fatigue loading for the web?
Thank you all in advance for your thoughts in this matter!

 

[rb1957]

1) think about principal stresses ... a web with in-plane shear has cracking across principal tension stress (so mode 1) You can take the transverse compression (along the crack) into account ... I think Rooke and cartwright have this.

2) I assume this is a structural bulkhead. Can you rationalise that is loaded by pressure cycle ('cause you say it isn't pressurised) ? Can you rationalise that it isn't loaded by gust/manoeuvre cycles ? Is it loaded by something else, like landing impact ??

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[B767]

Thank you rb1957 for your thoughts. Yeah, the plane is 737, 2.5g plane, at STA 540 bulkhead, (which is the center wing front spar, so it is not fwd or aft pressure bulkheads. Was thinking to come up with material capability based stress, I think principal stresses approach is valuable. Thanks again!

 

[rb1957]

Are you repairing the web ?

This sounds like a "spar frame" ... a fuselage frame supporting the wing front spar. Probably highly loaded, certainly complex fatigue spectrum. Not to be taken lightly. If you're repairing damage, I'd forward the question to Boeing.

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[B767]

...yes, it is a web repair, absolutely not taking it lightly!!!

 

[rb1957]

GL ! what are you doing for a fatigue spectrum ? limit load 1/flt ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[B767]

didn't go there yet, but yes, GAG, once per flight cycle, and limit is maybe too conservative!

 

[rb1957]

how are you determining the 1g flight load ? Is there a large pressure component ? wouldn't there be a gust/manoeuvre component ?

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[B767]

...well, I don't think pressure component is in the picture. and this a a plane over 100,000 lbs MTOW, so 14 CFR 337 and 341, for gust and maneuver, in this case maneuver is leading factor 2.5g

 

[WKTaylor]

First a curiosity question... I am uncomfortable NOT KNOWING what Your abbreviation SSI [subject line]. PLEASE ADVISE!!

Do You have access to stress analysis?

Without specific stress analysis that includes the frame, You will have to rely-on 'liaison/service-engineering' design principles/practices to develop a suitable repair. THEN stress will have 'something to analyze' in context of 'strength/stiffness equivalence' relative to the existing frame web, doublers, splices and stiffeners.

You are lucky, since this repair does NOT include pressurization loads.

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

 

[B767]

...my apology, SSI - Structural Significant Item

 

[rb1957]

"for gust and maneuver, in this case maneuver is leading factor 2.5g" ... huummm ... that's the limit nz for vertical manoeuvre, no?
not related to fatigue gusts/manoeuvre spectrum. If you're saying you're applying 2.5g as a fatigue load that may be sufficient/conservative ... but not sure how you'd calculate the front spar load for this, but NMP (Not My Problem).

"Hoffen wir mal, dass alles gut geht !"
General Paulus, Nov 1942, outside Stalingrad after the launch of Operation Uranus.

 

[B767]

rb1957, you're right, in Rooke&Cartwright, Fig 9, gives detail for stress intensity KII/Ko for Mode II cracks. Thanks a lot for that hint!

 

[WKTaylor]

Is this bulkhead 'built-up/riveted sheet metal'... or a 1-piece-part, machined from thick plate??

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

 

[B767]

WKTaylor, it is a 1 piece, machined down from 0.750' thick plate

 

[Ng2020]

Sounds dicey. Why can't you involve the OEM?
Do you have some professional stress engineering guidance available aside from EngTips?

 

[WKTaylor]

Hmmmmm... RE Your bulkhead...

It seems like it was machined from awfully thin plate stock. I was expecting at least 1.0-inch... up to +2.0-inch thick plate stock. This would have allowed for material removal both sides [surface 'clean-up]... and development of integral stiffeners. Is the 'web' at the min-plane [plate center]... with short [stubby] integral stiffeners? How large [X x Y] is it?

Do You have the drawing for the bulkhead... or is 'what-you-see-is-what-you-have'... other than structural repair manual [SRM] data?

What alloy-temper was the plate stock?

Was the finish machined part 100% shot peened [glass, ceramic or steel shot?]... or was it left 'bare' prior to finishing?

Was it anodized or was it 'alodined' [chromate conversion coating] or 'other'. What primer finish was applied?

Are the cracks originating at a penetration [fastener hole or cut-out opening] or at/along a integral stiffener/flange fillet radius? IE: are the cracks thru a stiffener fillet [~in-plane-with/parallel-to the plate thickness], or do they [also?] penetrate thru the web-thickness [transverse to the plate thickness] or both? How long is/are the crack/cracks?

Regards, Wil Taylor
o Trust - But Verify!
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation, Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", HBA forum]
o Only fools and charlatans know everything and understand everything." -Anton Chekhov

 

[LiftDivergence]

A couple of things...

1. Which sub-model are we dealing with here? From my experience with the 737-300/400/500 the STA 540 bulkhead is an oversized frame where the "web" would actually be defined as part of the wing center section structure. So I'm unclear as to what structure you're talking about specifically.

2. If you are working a STA 540 bulkhead repair, be careful of interference with SSID F-25.

3. I agree that "maneuver" conditions are likely boundary points (min / max) of the GAG cycle. For example, from experience with Book II I might expect something like approach + gust condition to define the max stress and taxi + bump to define the min stress. Not sure exactly, it depends on what exactly you're dealing with.

While the bulkhead web is obviously intended for shear loads, there will be some non-negligible induced load from pressurization of the fuselage. The pressure differential is minimum on the ground so the stress in between min and max in the GAG cycle will include some portion from internal pressure. That's fine if you just want to define a simplified spectrum based on the GAG stress.

However, based on what I've said it's not as if the GAGDR of the absolute min and max stress points will be 1.0. There might be some small amount of damage from spectral exceedances which would normally be accounted for by applying a spectrum factor to your nominal envelope limit stress in the GAG cycle.

If you plan on using absolute limit stress which would normally define residual strength requirements as the crack driving stress, I think you'd still want analytical confirmation that nominal peak * spectrum factor < unfactored static limit.

4. Whatever you do, I'd strongly suggest you look through the SRM section 51-70-13 typical web repairs. That won't really help you for DTE because they don't have inspections defined for that in the SRM.

Keep em' Flying
//Fight Corrosion!

 

[B767]

LiftDivergence, great input into this conversation, Thank You! Also, thank you all again for your thoughts. A two DER's with huge experience are involved already, a great opportunity for me to increase my knowledge and confidence.